Most pistons are connected to a rigid, mechanical link, such as a connecting rod connected to a crank shaft, and are therefore confined to preselected end limits. However, many machines are known which utilize one or more free pistons. A free piston reciprocates in a cylinder without such mechanical connection and therefore a free piston is not mechanically confined to end limits. Such free pistons may be driven by an electromagnetic, linear motor and used, for example, as a gas or other fluid compressor or pump. Free pistons are also found in free piston Stirling cycle machines, such as free piston Stirling engines and cryocoolers. Free pistons sealingly reciprocate in a cylinder formed in a housing. The housing typically encloses a work space bounded by one end of the piston and a second space or back space bounded by the opposite end of the piston. The free piston makes excursions from a mean position in opposite directions to a bottom dead center (BDC) position and a top dead center (TDC) position. The amplitude of these excursions from the mean position varies as a function of the operating conditions of the machine, such as the work demand, load or gas pressures. The piston displacement from the mean position is approximately a sinusoidal function of time or angle. The amplitude of a piston's reciprocation is the displacement from the mean position to the BDC position or to the TDC position. The distance from the BDC position to TDC position is the piston's stroke.
Not only do changes in the operating conditions of the machine cause variations in the amplitude of the piston excursions, but such changes also cause variations or creep of the mean position. Variations in the mean position may result, for example, from non-symmetrical variations of the pressures to which the piston is exposed as a function of time and the resulting non-symmetrical leakage of working fluid past the piston seals.
Typically the first or work space and the second or back space volumes experience fluid pressure variations as a function of time about an average pressure. Typically the back space pressure variations as a function of time are smaller and more nearly sinusoidal than the pressure variations in the work space. The result of assymmetrical pressure variations as a function of time in the work space and the non-symmetrical leakage past the piston, is to cause a net leakage of working fluid from one space to the other, most commonly from the work space into the second or back space. Thus, although during each cycle minute quantities of gas will leak first in one direction and then in the other, the leakages in opposite directions are typically not equal and therefore result in a small net transfer of gas from one space to the other during each cycle. This gas transfer between the spaces gradually accumulates and consequently causes the mean position of the free piston to creep toward one end or the other, typically creeping inwardly toward the work space.
In summary, although both the first and second spaces each experience pressure variations as a function of time, about an average pressure, a net leakage in one direction, such as from the first space to the second space, causes a net migration or creep of the mean position of the piston in the opposite direction, such as from the second space toward the first space. Thus, even if the amplitude of oscillation remains constant, a sufficient creep may eventually cause the piston to collide with a bounding structure, such as the valve plate of a compressor, the heat exchanger of a Stirling engine or the displacer of a Stirling engine. Similarly, even if the mean position remains unchanged, an increase in the amplitude of oscillation, caused for example by a reduced load demand, can likewise cause such a collision.
There is therefore a need for an end limiting structure which limits the end position of the reciprocating free piston in order to maintain a selected clearance between the piston and other bounding structures. For example, in a free piston compressor it is desirable that the piston approach the valve plate as closely as possible at a minimum clearance, sometimes on the order of hundredths of a millimeter, in order to operate the compressor at a maximum compression ratio and therefore at maximum efficiency. However, it is also necessary to assure that the piston cannot travel any further toward the valve plate so that destruction of the machine by collision with the valve plate is avoided. Alternatively, it is sometimes desirable for thermodynamic reasons to limit the end position of a free piston.
It is a further object and feature of the present invention to maintain the end limit for the piston position over a wide range of stroke or amplitude of piston reciprocation, as well as over a wide range of mean position for the reciprocating piston as operating conditions vary, such as by a demand for a greater or lesser refrigerant flow rate, a change in refrigerant pressure or a demand for more or less work by a Stirling engine or a demand for a higher or lower thermal pumping rate by a Stirling cooler.